JPS6137890A - El element - Google Patents

Abstract

PURPOSE:An inexpensive EL element having a good light emission efficiency by low-voltage driving, which is made by holding a light-emitting layer of an electron-accepting, electroluminescent org. compd. and a light-emitting layer of an electron-donating, electroluminescent org. compd. between two transparent electrode layers. CONSTITUTION:A light-emitting layer 2, 0.02-1mum in thickness, comprising a first light-emitting layer 4, 0.01-0.5mum in thickness, consisting of a deposited molecular film of an electroluminescent org. compd., such as carbazole, which is more electron-accepting than a second light-emitting layer and the second light- emitting layer 5, 0.01-0.5mum in thickness, consisting of a deposited molecular film of an electroluminescent org. compd., such as anthracene, which is more electron-donating than the first light-emitting layer, is held between two electrode layers 1 and 3 of which at least one is transparent.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to an EL element using electrical light emission, that is, EL, and more specifically, the present invention relates to an EL element using electrical light emission, that is, EL, and more specifically, the light emitting layer has a two-layer structure,
The present invention relates to an EL device comprising a thin film of at least one electroluminescent organic compound, each layer having a different electronegativity relative to other adjacent layers.

(Prior art) Conventional EL elements are made of Mn, Cu or Re F3.
ZnS containing (Re; rare earth ion) etc. as an activator
It consists of a light-emitting layer having a light-emitting base material, and is structurally classified into powder-type EL and thin-film type EL, depending on the basic structure of the light-emitting layer.

Among devices that have been put into practical use, thin-film ELs generally have higher brightness than powder-type ELs, but because thin-film ELs form a light-emitting layer by vapor-depositing a light-emitting base material onto a substrate, they are not suitable for large-area devices. It has the drawbacks that it is difficult to manufacture and the manufacturing cost is very high.

For this reason, powder-type EL, in which a light-emitting base material, that is, ZnS, is dispersed in an organic binder, which is most easily mass-produced and costs only a few tenths of the cost of thin-film devices, has attracted attention. Generally, in EL light emission, the thinner the thickness of the light emitting layer, the better the light emission characteristics. However, in the case of the powder type EL, since the luminescent base material is a discontinuous powder, pinholes are likely to occur in the luminescent layer when the luminescent layer is thinned.
It has a major drawback in that it is difficult to reduce the layer thickness, and therefore sufficient brightness characteristics cannot be obtained. Even in the case of slow sowing, an improved element in which an intermediate dielectric layer made of vinylidene fluoride polymer is disposed within the light-emitting layer of the powder type EL is disclosed in JP-A-58-172891.
It has not yet achieved sufficient performance in terms of luminance, power consumption, etc. On the other hand, recently, research and development has been actively conducted to control the chemical structure and higher-order structure of organic materials to create new materials for optical and electronics.
Organic materials, such as piezoelectric elements, pyroelectric elements, nonradiometer optical elements, and ferroelectric liquid crystals, have been announced that are comparable to or superior to metals and inorganic materials. As described above, there is a demand for the development of functional organic materials as new functional materials that surpass inorganic materials, and a cumulative film of monomolecular layers of anthracene derivatives and pyrene derivatives, which have a parent tree group and a hydrophobic group in the molecule, is being used as an electrode. The EL element formed on the substrate was disclosed in Japanese Patent Application Laid-Open No. 52-35587.
It is proposed in the Publication No. However, these EL devices have not achieved sufficient performance in terms of brightness, power consumption, etc. as actual EL devices, and furthermore, in the case of organic E'L devices, the density of carrier electrons or holes is extremely low. The probability of excitation of functional molecules due to carrier recombination is extremely small, and efficient light emission cannot be expected.

(Disclosure of the Invention) Therefore, an object of the present invention is to solve the above-mentioned drawbacks of the prior art, and to provide an EL element that can emit light with sufficiently high brightness even when driven at a low voltage, is inexpensive, and is easy to manufacture. It is to provide.

The above object of the present invention has been achieved by forming a light emitting layer of an EL element by combining specific materials and having a specific configuration.

That is, the present invention provides an EL device comprising a two-layered light emitting layer and two electrode layers sandwiching the light emitting layer, at least one of which is transparent. A molecular deposited film comprising at least one electroluminescent organic compound that is electron-accepting relative to the second emissive layer, the second emissive layer being electron-donating relative to the first emissive layer. The above EL device is characterized in that it is made of a molecular deposited film made of at least one electroluminescent organic compound.

To explain the present invention in detail, the electroluminescent organic compound used in the present invention and which mainly characterizes the present invention has a high luminescence quantum efficiency and is easily susceptible to external perturbation.
It is a compound that has an electronic system and can be electrically excited.1 For example, basically fused polycyclic aromatic hydrocarbons, p-terphenyl, 2.5-diphenyloxazole, 1.4-
Bis(2-methylstyryl)-benzene, xanthine,
Examples include coumarin, acridine, cyanine dyes, benzophenone, phthalocyanine and its metal complexes, porphyrins and its metal complexes, 8-hydroxyquinoline and its metal complexes, organic ruthenium complexes, organic rare earth complexes, and derivatives of these compounds. Wear. Furthermore, compounds that can serve as electron acceptors or electron donors for the above compounds include heterocyclic compounds other than those mentioned above and derivatives thereof, aromatic amines and aromatic polyamines, compounds with a quinone structure, and tetracyanoquinodimethane. and tetracyanoethylene.

Examples of the basic skeleton of the compound and other compounds useful for forming the light-emitting layer in the present invention are as follows. (However, the basic skeleton illustrated below has 1 to 4 carbon atoms.
Alkyl group, alkoxy group, alkyl ether group, halogen atom, nitro group, primary to tertiary amine group, hydroxyl group,
It may have common substituents such as carboxamide groups and sulfoamide groups. ) (Margin below) Z=NH10, S Z=CO, NHZ=CO1NH
,O,5Z=NH,O,5 Z=NH1O,S Z=NH,0
,5Z-81Se Z-81se Z-
81seZ=NH10,SZ-NHlQSZ-NI
(, O%SM=Mg, Zn5sn1AICI M
=Hz, BetMgiCatCdSn, AZCI, Yb
CI M= Er, Trrx Sm, Eu, Tb,
Z = O, N group u M-A4 Ga i I r s Ta s a=3
M≠Er, Sm, EuM=Zn, Cd%Mg,
pb, a=2 Gd, Tb, DyTm
, yb M=Er, Sm, Eu, Gd M=Er, Sm
, EuTb, Dy, Tm, Yb G
d, Tb, DyTm, Yb Z=0. S, Se O≦p≦2 The above luminescent compounds can be used alone or as a mixture in each luminescent layer in the present invention. In addition,
These compounds are examples of preferred compounds, and it goes without saying that other derivatives or other compounds may be used as long as the same purpose is achieved.

The present invention is characterized in that the luminescent compounds as described in - are used separately in the first luminescent layer and the second luminescent layer of the EL device of the present invention according to their electronegativity. . That is, since the luminescent compounds shown in - have different electronegativities, among them, the compounds that are relatively electron-accepting are selected as the luminescent compounds for forming the first luminescent layer. The light-emitting compound that is relatively electron-donating to them may be selected as the second light-emitting layer forming compound.

Among such luminescent compounds, particularly preferable electron-donating compounds are those having electron-donating groups such as primary to tertiary amino groups, hydroxyl groups, alkoxy groups, alkyl ether groups, or nitrogen-donating compounds. Heterocyclic compounds are the main ones, and electron-accepting compounds are mainly compounds having electron-withdrawing groups such as carbonyl groups, sulfonyl groups, nitro groups, and quaternary amine groups. In the present invention, such luminescent compounds can be used alone or in combination in each luminescent layer.

The other elements forming the EL element of the present invention, namely the transparent electrode layer and the back electrode layer, sandwich the light emitting layer, and
Any conventionally known material can be used, but at least one layer thereof must be transparent. As the transparent electrode layer, any desired transparent electrode layer can be used as in the past, and preferred examples include polymethyl methacrylate,
A transparent conductive material such as indium oxide, tin oxide, or indium tin oxide coated entirely or in a pattern on the surface of a transparent film or sheet such as transparent synthetic resin such as polyester or glass. It is. When using an opaque back electrode layer on one side, these opaque electrode layers may also be conventionally known ones.
A typical and preferred thickness is about 001 to 0.31
It is a vapor deposited film of aluminum, silver, gold, etc. of Lm. Further, the shape of the transparent electrode layer or the back electrode layer may be any shape such as a plate, a belt, or a cylinder, and can be selected depending on the purpose of use. Furthermore, the thickness of the transparent electrode layer is approximately 0.0
A thickness of about 1 to 0.2 m is preferable. If the thickness is less than this range, the physical strength and electrical properties of the element itself will be insufficient, and if the thickness is more than the above range, it will not be transparent, lightweight, or compact. There is a risk that problems may occur.

In the EL device of the present invention, - a luminescent layer consisting of two layers is formed by separately using electroluminescent compounds having relatively different electronegativities as described above between the two electrode layers as described above; The first layer constituting the formed two-layer structure light-emitting layer is a molecular deposited film made of a compound that is electron-accepting relative to the second layer, and the second layer The first layer is a molecular deposited film made of a compound that is electron-donating relative to the first layer, and the chin is characterized by a molecular deposition film.

In the present invention, a particularly preferable method for forming the molecular deposition films constituting the first and second light emitting layers is as follows:
The method is a resistance heating vapor deposition method or a CVD method. For example, in the vapor deposition method, a thin film of about 500 A can be formed as the first and second light emitting layers.

For example, when using the resistance heating vapor deposition method, the material is placed in a tungsten board placed in a vacuum chamber, and the material is placed 30 cm from the substrate.
Otherwise, resistance heating is performed, and sublimable materials are set at the sublimation temperature, and meltable materials are vapor deposited at a temperature higher than the melting point. The pre-vacuum level is set to 2 x 10 Tarr or less, the port is closed with a shutter before vapor deposition, the port is heated and the air is allowed to air for 2 minutes, and then the shutter is opened to perform vapor deposition.

The speed during deposition is measured using a crystal oscillator film thickness monitor, but a suitable speed is 0°IA/sec~
It is carried out at a rate of 100 A/sec. The degree of vacuum at that time is 1 O Torr or less, preferably 1 O Torr, to prevent oxidation etc.
This is done by maintaining the pressure at about Torr.

In the EL element of the present invention, among the two electrode layers as described above,
As the first layer, a compound that is relatively electron-accepting is selected from the above-mentioned compounds, and as a second layer, a compound that is relatively electron-donating with respect to the first layer is selected from among the above-mentioned compounds. This can be obtained by selecting , forming a molecular deposition film in the same manner, and forming the light emitting layer into a two-layer structure.

In the prior art, it is known that an EL device is formed by a molecular deposition method, but this known method cannot produce an EL device with sufficient performance, and as a result of various research, the inventor has a two-layer structure, the first layer, the light-emitting layer, is formed as a molecular deposition film using a relatively electron-accepting compound as described above, and the second layer is formed with a relatively electron-accepting compound as described above. It has been discovered that the performance of conventional EL devices can be significantly improved by similarly forming a molecular deposition film from a compound that is electron-donating.

The thickness of the light-emitting layer of the EL device of the present invention obtained in this way can be changed arbitrarily, but in the present invention, the thickness of the first layer is set to about 0.01 to 0.5 mm. and second
Preferably, the layer thickness is between about 0.01 and 0.5 gm, and the overall thickness of the emissive layer is between about 0.02 and 1 μm.

Note that the adhesion between one or both electrode layers used as a substrate and the light-emitting layer is sufficiently strong in the molecular deposition method, and the light-emitting layer will not peel or fall off. In order to strengthen the adhesive force, the surface of the substrate may be treated in advance, or a suitable adhesive layer may be provided between the substrate and the light emitting layer.

The performance of the EL element formed as described above may deteriorate due to the influence of moisture and oxygen in the air if left as is, so it is desirable to create a moisture- and oxygen-proof hermetically sealed structure using conventionally known means. .

In the EL device of the present invention as described above, the structure of the light emitting layer is an ultra-thin film, and the first layer and the second layer have various electrical interactions, so that excellent light emission can be achieved. It is something that demonstrates performance.

Furthermore, as schematically shown in FIG. 1, the light-emitting layer of the EL device of the present invention is different from the light-emitting layer consisting of a single layer in the prior art, as schematically shown in FIG. light emitting layer and second
Since the light-emitting layer has a uniform interface, various interactions between these two layers with different electronegativity are extremely easy, and it exhibits excellent light-emitting performance that cannot be achieved with conventional technology. It is something to do. That is, by variously changing the difference in electronegativity between the first light-emitting layer and the second light-emitting layer, the light emission intensity can be improved or the light emission color can be changed arbitrarily, and the service life can also be changed. It can be significantly extended.

Furthermore, in the conventional technology, materials with excellent luminescence properties but insufficient film formability or film strength could not be practically used.
In the present invention, even if such a material has poor film formability and film strength but has excellent luminescent properties, by using a material with excellent film forming properties for either layer, the luminescent properties and film forming properties can be improved. A light-emitting layer having excellent properties and film strength can be obtained.

The EL device of the present invention described above can achieve excellent EL performance by applying electrical energy such as alternating current, pulse, or direct current between the electrode layers so that electrical energy such as a suitable electric field acts on the light emitting layer. It shows luminescence.

Next, the present invention will be explained in more detail with reference to Examples. Note that the words in the middle of the text are based on weight.

Example 1 A transparent electrode was formed by depositing an ITO layer with a thickness of 1500 nm on the surface of a 50 mm square glass plate by sputtering.

Next, carbazole (A) (mp, 245°C) was deposited on the transparent electrode substrate using a resistance heating evaporation device.
was deposited to a film thickness of 550 mm. This vapor deposition is carried out by first reducing the pressure in the vapor deposition tank to a vacuum level of 10 Torr, and gradually increasing the temperature of the resistance heating board (M o ) so that the vapor flows to the board containing carbazole at a vapor deposition rate of 5 A/sec. A deposited film was formed by adjusting the current. The degree of vacuum during thin deposition was 9×10-' Tarr. Also, the temperature of the substrate holder is 20'C! water was circulated and kept constant.

Next, after returning the vapor deposition tank to normal pressure, the vapor deposition compound was exchanged with Anthracef (B) (mp, 216°C).

After reducing the pressure in the deposition tank to a vacuum level of 10 Torr, the temperature of the resistance heating board (Mo) was adjusted so that the vacuum level was 9 x 10-5 Tarts, and the deposition rate was 5λ/se.
In step c, anthracene was deposited on the deposited film to a thickness of 550 nm. The board material was Mo as described above, and the degree of vacuum during vapor deposition was 9 X I O-'T orr.

Further, the temperature of the substrate holder was kept constant by circulating 20'0 water.

Finally, the thin film formed as described above was placed in a deposition tank, and the nuclide was once depressurized to a vacuum level of 10-' Torr, and then the vacuum level was adjusted to 10 Torr, and the deposition rate was 20 A.
/ See Te, A1 was evaporated onto the thin film to a film thickness of 1500λ to form a back electrode. As illustrated in FIG. 3, the produced EL device is sealed with a sealing glass, and then purified, degassed, and dehydrated silicone oil is injected into the seal according to a conventional method to produce an EL light-emitting cell of the present invention. Formed. When an AC voltage of 20 V and 400 Hz was applied to this EL light emitting cell, the current density was 0.08 mA/c♂
It showed full EL emission with a brightness of 20 ft-L.

Note that the above EL element can also be produced by a CVD method or the like.

The above-mentioned EL element of the present invention had a lower driving voltage and better luminance characteristics than the conventional EL element using ZnS as a light emitting matrix.

Comparative Example 1 A comparative EL element was obtained in the same manner as in Example 1 except that the first layer was not formed,
Moreover, when evaluated in the same manner as in Example 1, the current density was 0,
1 mA/cm't' power was less than 10 ft-L.

Example 2 The following compounds C and D were used in place of compounds A and B in Example 1, and the EL device of the present invention was obtained in the same manner as in Example 1 except for CD. When evaluated, the current density was 0, 1
At 1 mA/cm2, the luminance (Ft-L) was 2B.

[Brief explanation of drawings]

FIG. 1 diagrammatically shows an EL device according to the prior art, FIG. 2 diagrammatically shows an EL device of the present invention, and FIG. 3 diagrammatically shows an EL device of the present invention. This is a diagrammatic illustration of a cross section. 1; Transparent electrode 2: Luminescent layer 3: Back electrode 4; Luminescent compound 5; Luminescent compound 6; Seal glass 7; Silicon insulating oil 8
Nigarasu Plate Figure 1 Figure 2

Claims (1)

[Claims] A two-layer structure light-emitting layer and at least one light-emitting layer sandwiching the light-emitting layer.
In an EL device comprising two transparent electrode layers, the first emissive layer is made of at least one electroluminescent organic compound that is electron-accepting relative to the second emissive layer. The second light-emitting layer is a molecular deposited film made of at least one electroluminescent organic compound that is electron-donating relative to the first light-emitting layer. The above EL element.